Prion diseases represent a category of neurodegenerative disorders. Prion diseases of humans and domestic animals include Creutzfeldt-Jakob disease (OD) in humans, scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease in deer and elk, transmissible encephalopathy in mink, and spongiform encephalopathy in domestic and feral cats (Silveira et al., Curr. Top. Microbiol. Immunol. (2004) 284:1-50). These diseases are transmitted primarily by the oral route and it appears initiation and progression of the disease is based on the misfolding of a normal cellular protein (PrPC) into a self-propagating and infectious conformation, (PrPSc).
PrPSc is thought to serve as a template to recruit normal cellular prion protein (PrPC) to the infectious form of PrPSc in an autocatalytic process (Prusiner S. B., Proc. Nat. Acad. Sci. USA (1998) 95:13363-13383). Transmission of PrPSc within a population is believed to occur primarily through either environmental contamination by infected body fluids or by recycling of animal proteins within the food chain. For example, BSE is believed to be the result of cannibalism in which faulty industrial practices produce prion-contaminated feed for cattle. There is now considerable concern that bovine prions may have been passed to humans, resulting in a new form of CJD.
Efforts to eliminate BSE transmission have focused primarily on the removal of animal protein from ruminant feeds (Smith et al., Br. Med. Bull. (2003) 66:185-198). However, the occurrence of BSE in cattle following the elimination of feed exposure has raised concerns regarding persistent environmental contamination by PrPSc. The persistence of PrPSc in the environment means that new strategies are required to minimize the risk of animal exposure or infection when animals are housed in a contaminated environment. Furthermore, strategies may be required to eliminate or minimize the risk of a PrPSc shedding into the environment.
The pathogenesis of scrapie and BSE has been studied using a variety of experimental models and it is evident that prion diseases can be orally transmitted in sheep (Heggebo et al., J. Gen. Virol. (2000) 81:2327-2337), cattle (Terry et al., Vet. Rec. (2003) 152:387-392) and possibly cervids (Sigurdson et al., J. Gen. Virol. (2001) 82:2327-2334). These animal models have consistently revealed that following oral challenge, PrPSc can be detected in mucosa-associated lymphoid tissues (MALT) which include the tonsils, and Peyer's patches within the small intestine. It appears that these tissues function as a primary site of prion propagation before the PrPSc is transported by splanchic innervation to the brainstem and spinal cord (Sigurdson et al., Br. Med. Bull. (2003) 66:199-212; Marsh et al., Rev. Sci. Tech. (1992) 11:539-550).
The follicular dendritic cell, which is present within the lymphoid follicles of MALT, lymph nodes, and the spleen, appears to be of particular importance in the peripheral propagation of PrPSc (Mabbott et al., J. Virol. (2003) 77:6845-6854). These cells are long-lived, express high levels of PrPC, and are specialized to trap, retain, and present antigen. The tingible body macrophage, which is also present within lymphoid follicles, may also play a role in regulating prion disease by degrading PrPSc (Jeffrey et al., J. Pathol. (2000) 191:323-332). Thus, lymphoid tissues provide a critical link in the pathogenesis of prion diseases and may also provide a site where the balance between the generation and destruction of PrPSc may be altered through the application of immunoprophylaxis.
Several investigations have confirmed that antibodies directed against PrPC can clear cell lines infected with PrPSc (Enari et al., Proc. Natl. Acad. Sci. USA (2001) 98:9295-9299; Peretz et al., Nature (2001) 412:739-743). These observations have been interpreted as evidence that antibodies may be able to interfere with the intermolecular interactions between PrPC or the compartmental cycling of this protein and thereby disrupt the conversion of PrPC to PrPSc. However, antibodies directed against PrPC, a normal cell surface protein, may have adverse consequences in vivo. For example, circulating antibodies against PrPC may trigger complement-dependent lysis of cells or possibly induce autoimmune disease by breaking tolerance to this molecule. Alternatively, antibodies directed against PrPC may impair or alter the function of this normal cellular protein by either triggering apoptosis in neurons (White et al., Am J. Pathol. (1999) 155:1723-1730) or inappropriate activation of cell signaling cascades (Cashman et al., Cell (1999) 61:185-192; Mouillet-Richard et al., Science (2000) 289:1925-1928).
It has been demonstrated that the conversion of recombinant mouse prion protein to a β-sheet enriched form is accompanied by increased solvent exposure of tyrosine (Y) side chains (Paramithiotis et al., Nat. Med. (2003) 9:893-899). Six of the 11 Y residues in the protease-resistant core region of PrPSc are present in conserved bi-tyrosine pairs in mouse, hamster, sheep, bovine, human, elk, mule deer and whitetail deer PrP. Two bi-tyrosine pairs [YY], located in the helix one and β-strand two, are in conjunction with a C-terminal arginine [R] which creates a YYR motif (FIG. 1). U.S. Pat. No. 7,041,807 describes rabbit polyclonal antisera raised against YYR peptide and immunoprecipitation of PrPSc from scrapie-infected mouse brain but did not PrPC from uninfected brains. Furthermore, a series of murine monoclonal antibodies (mAbs) specific for the YYR motif were reported as capable of immunoprecipitating PrPSc from scrapie infected mouse and hamster brains, scrapie infected sheep brain, and BSE-infected bovine brain. These anti-PrPSc antibodies were raised using YYR peptide formulated in conventional adjuvants in animals expressing endogenous PrPC. Furthermore, neither the rabbit polyclonal antisera or the murine mAbs displayed a detectable reaction with surface proteins on normal cells.
In spite of the considerable potential of the YYR epitope in generating antibody responses that are specific to PrPSc its application as a vaccine target is restricted by the limited immunogenicity of this motif. As PrPC is widely expressed, and in particular at the surfaces of B and T cells, wild-type animals are highly tolerant of antigens of PrPC making it difficult to induce potent antibody responses. This is of particular importance as the ability for antibodies to function as prion therapeutics is directly related to their titer (White et al., Nature (2003) 422:80-83). As such, the ability to induce robust and sustained antibody responses is believed to be a critical determinant of vaccine efficacy.
Thus, there remains a need for the development of effective strategies for the treatment, prevention and diagnosis of prion infection.